1 Technical Field
The present invention relates to an assembly of a spark plug and an engine main body, which includes a gasket to hermetically seal between the spark plug and the engine main body.
2 Description of Related Art
A spark plug is generally mounted to a main body of an internal combustion engine of a motor vehicle or a cogeneration system, so as to ignite the air-fuel mixture in a combustion chamber of the engine by generating sparks in a spark gap of the spark plug.
More specifically, the spark plug is generally mounted to a cylinder head (i.e., a part of the main body) of the engine by threadedly engaging a male-threaded portion formed on an outer surface of a metal shell of the spark plug with a female-threaded portion formed in an inner surface of a spark plug-mounting bore of the cylinder head.
Moreover, there is disclosed, for example in Japanese Patent Application Publication No. 2001-187966, a technique for reliably sealing between the metal shell of the spark plug and the cylinder head. According to the technique, a substantially annular gasket is interposed between a seat surface of the cylinder head and a seating surface of the metal shell of the spark plug. The seat surface of the cylinder head is formed around an open end of the spark plug-mounting bore; the open end is on the opposite side to the combustion chamber of the engine. The seating surface of the metal shell is formed on the proximal side (i.e., the opposite side to the combustion chamber) of the male-threaded portion of the metal shell so as to face the seat surface of the cylinder head. In mounting the spark plug to the cylinder head, the male-threaded portion of the metal shell of the spark plug is tightened into the female-threaded portion of the cylinder head, elastically deforming the gasket interposed between the seat surface of the cylinder head and the seating surface of the metal shell. Consequently, with an elastic force of the gasket, which is created by the elastic deformation of the gasket, it is possible to maintain the tightening axial force of the male-threaded portion of the metal shell of the spark plug, thereby forming a hermetic seal (or fluid-tight seal) between the seat surface of the cylinder head and the seating surface of the metal shell.
However, in recent years, lean burn and high output have been pursued for engines, resulting in increases in the combustion temperatures as well as in vibration of the engines. Consequently, when the above technique is used, an excessive force may come to be applied to the gasket, causing the gasket to be plastically deformed and thereby decreasing the thickness of the gasket in the axial direction of the spark plug. That is, “permanent set” of the gasket may occur, thereby lowering the elastic force of the gasket. As a result, it may become difficult to secure a high sealing performance between the metal shell of the spark plug and the cylinder head of the engine.
Further, with occurrence of permanent set of the gasket, the tightening axial force of the male-threaded portion of the metal shell of the spark plug may be lowered, thereby loosening the engagement between the male-threaded portion of the metal shell and the female-threaded portion of the cylinder head.
To prevent permanent set of the gasket from occurring, one may consider increasing the yield stress of the gasket.
On the other hand, the main purpose of employing the gasket is to realize, with the elastic deformation of the gasket during the tightening of the male-threaded portion of the metal shell into the female-threaded portion of the cylinder head, intimate contact between the seat surface of the cylinder head and the seating surface of the metal shell, thereby securing a high fluid-tightness therebetween.
However, if the yield stress of the gasket is increased for preventing occurrence of permanent set of the gasket, it may become difficult for the gasket to be elastically deformed during the tightening of the male-threaded portion of the metal shell into the female-threaded portion of the cylinder head, thereby making it difficult to realize intimate contact between the seat surface of the cylinder head and the seating surface of the metal shell. Consequently, it may become difficult to secure a high fluid-tightness between the seat surface of the cylinder head and the seating surface of the metal shell.